Insulator body having an electrically conductive surface and method



Aug. 13, 1968 M.'E. POLA INSULATOR BODY HAVING AN ELECTRICALLYCONDUCTIVE SURFACE AND METHOD Filed April 14, 1965 INVENTOR. MIL TEN E.PULAND.

BY QWJQMH ATT D's.

United States Patent "ice 3,397,083 INSULATOR BODY HAVING ANELECTRICALLY CONDUCTIVE SURFACE AND METHOD Milton E. Poland, Royal Oak,Micl1., assignor to Champion Spark Plug Company, Toledo, Ohio, acorporation of Delaware Filed Apr. 14, 1965, Ser. No. 448,188 11 Claims.(Cl. 117-201) ABSTRACT OF THE DISCLOSURE A method of producing anelectrically conductive surface on an insulator body consistingprincipally of alumina which includes the steps of applying to a surfaceof the insulator a coating of a copper oxide containing compositioncapable of forming an electrically semiconducting on the insulator, thecomposition including at least about 3 percent of an oxide of chromium,the amount of the oxide being sufficient to stabilize the surfaceresistance of the coating upon firing to temperatures aboveapproximately 2400 F., and firing the insulator and coating to atemperature above approximately 2400 F., but not sufficiently high thatthe surface resistance of the coatin is appreciably higher than thesurface resistance of the coating fired to maturation, and for a timesufficiently short that, adjacent the original insulator surface andimmediately therebelow there is a copper rich electricallysemiconductive region which is substantially devoid of free alumina.Also disclosed is an insulator body produced in accordance with theabove method.

The present invention relates to insulator bodies having a surface layerof a semiconductive material thereon; and more particularly to sparkproducing devices having spaced-apart electrodes in contact with asemiconductive surface layer of an insulator body as occurs, forexample, in jet igniters.

It has been common practice heretofore to use a semiconductive materialbetween and in abutment with the electrodes of jet engine igniters toreduce the onset voltage that is necessary to induce a spark between theelectrodes. In some instances, the prior art has used a disc ofsemiconductive material placed between and in abutment with theelectrodes, while in others the prior art has used a thin coating,commonly called an engobe coating, applied to an electrical insulatorbody in such manner that a thin semiconductive layer of the engobe abutsand bridges the gap between the electrodes of the igniter. The insulatorbodies used in substantially all instances have contained approximately85 percent 1 to 95 percent of alumina, and have been fired into hard,erosion resistant bodies having but a few percent of voids therein. Theengobe coatings which have been used heretofore have an oxide of eitheriron or copper as the electrically conductive ingredient thereof. Theseingredients have usually been mixed with one or more other oxides.Perhaps the most commonly used material has been one comprising oxidesof copper mixed with a minor percentage of either chromic oxide orferric oxide or a mixture of both, and in some instances copper powdermixed with chromic oxide or ferric oxide and made into a slurry has beenapplied to the insulator body and fired in an oxidizing atmosphere toproduce an oxide of the copper in situ.

semiconductive material when used to bridge the spark gap of an igniterwill, of course, erode away. Discs of appreciable thickness can provideconsiderable material be- The terms percent and parts are used herein,and in the appended claims, to refer to percent and part by weight,unless otherwise indicated,

3,397,083 Patented Aug. 13, 1968 fore being eroded away by the spark butit has been found that the thickness of discs required to give longservice life do not confine the fiow of electricity to the surface ofthe semiconductor and, therefore, require too high a current flow beforesparking. It has also been found that prior art engobe coatings tend tobe soft so that they erode away quickly without providing acceptableservice life.

An object of the present invention is the provision of a new andimproved insulator body having a semiconductive surface layer which whenplaced into abutment with spaced-apart electrodes is highly resistant tospark erosion.

Another object of the present invention is the provision of a new andimproved insulator body of the abovedescribed type which includes asemiconductive layer at and just beneath the surface of a firedinsulator body, and formed by diffusion of semiconductive oxides intothe structure of the insulator body.

A further object of the present invention is the provision of a new andimproved insulator body having a surface semiconductive layer of theabove-described type wherein a minimum of the material of the originalinsulator body is diffused out of the insulator body during the timethat the conductive materials are diffused into the surface of theinsulator body.

A more specific object of the present invention is the provision of anew and improved insulator body having a semiconductive surface layer ofthe above-described type wherein the semiconductive material is aconductive aluminate formed in situ by reaction between the materials ofwhich the insulator body is made and another oxide diffused into thesurface of the insulator body.

A still more specific object of the present invention is the provisionof a new and improved alumina insulator body in which cuprous oxide isdiffused into the crystal lattice of the alumina adjacent the surface ofthe insulator body to form a dense aluminate without appreciablemigration of the alumina out of the surface of. the alumina body.

Another object of the present invention is the provision of a new andimproved method of making an insulator body having a semiconductivesurface layer thereon and wherein the surface of an alumina insulatorbody is coated with a slip comprising a minor amount of alumina and amajor amount of an oxide which will form an electrically conductiveauminate with the alumina of the insulator body and the coated body isthen heated to produce a dense electrically conductive aluminate in theinsulator adjacent its surface.

Other objects and advantages of the invention will be apparent from thedescription which follows, reference being had to the accompanyingdrawing, in which the sole figure is a sectional view through a jetengine igniter embodying principles of the present invention.

Although the invention has utility in various types of applicationswherein it is desired to make the surface of an insulator electricallyconductive, it has particular advantages when embodied in jet engineigniters wherein the electrically conductive material is subject tosevere mechanical and thermal shock conditions as well as corrosiveatmospheres and alternately reducing and oxidizing conditions. In orderthat a better understanding of the invention can be had as quickly aspossible, the invention will first be described in detail as embodied ina jet engine igniter, and thereafter some of the various modificationsof which the invention is capable will be explained.

As above mentioned a preferred application of the present inventionoccurs in a jet engine igniter. The jet engine igniter shown in thedrawing is designated by the numeral 11 and has a thin electricallysemiconductive layer which bridges two electrodes to provide a path forthe discharge of a high energy spark. The igniter 11 comprises a metalshell 12 threaded as at 13 for insertion into the combustion chamber,for example, of a jet engine, and threaded at 14 for reception of anignition harness. A ceramic insulator 15 attached to a second ceramicinsulalator 16 is sealed inside the metal shell 12, and a centerelectrode 17 is sealed inside the insulator 15. An electricallysemiconductive layer produced according to the invention is provided onthe nose end of the insulator 15 and is designated by the numeral 18.The layer 18 provides an electrical path interconnecting the nose end 19of the center electrode 17 and a ground electrode 20 attached to theshell 12. When a comparatively low voltage charge is applied to thecenter electrode of the igniter 11, for example, from a condenser, ahigh energy electric discharge occurs along the surface of thesemiconductive layer 18. The electrical resistance of the layer 18 willin general depend upon its thickness as well as the material from whichit is made, and the resistance will decrease as the thickness of thelayer increases. The thickness of the layer 18 can be controlled by themanner of production.

The present invention is concerned primarily with the composition of thesemiconducting layer 18 as well as the method by which such layer isproduced. Accordingly, one of the best known modes of producing thislayer will be explained by means of the following example.

Example I A preferred embodiment of igniter was made using a previouslyfired ceramic insulator 15 containing approximately 92% of A1 0 fired toapproximately 95% of theoretical density. A slip comprising thefollowing materials was prepared by mixing with an equal weight of waterand was brushed on the lower end surface of the insulator 15:

Percent Copper metal powder 70.2 Chromic oxide 7.8 Alumina 20.0 Purekaolin 2.0

The insulator with the coating of the above-described slip was advancedthrough a furnace, the internal temperature of which was maintained at2500 F. in a manner which allowed the insulator to remain in the hotzone of the furnace for ten minutes. The coated insulator was thereafterallowed to cool to room temperature, another coating of theabove-described slip was brushed thereon and the re-coated insulator wasrefired for the same time and temperature described above. The coatedinsulator was again allowed to cool to room temperature, and a thirdcoating of the slip was applied and fired in the same manner as were thetwo previously described coatings.

In a development program of the type involved in the invention of theabove-described igniter, it is necessary that a great number ofspecimens be spark tested, and that a standard specimen and testarrangement be used for spark testing each insulator body having asemiconductive layer thereon. The test specimens which were used weretubular bodies 1% inch long and having an CD. of 0.355 inch and an ID.of approximately 0.100 inch. These insulator bodies were made of thesame material as the insulator 15, above-described, and comprisedapproximately 92% of alumina. Slips of various materials were brushedonto the lower annular surface of the test insulators and were thenfired at a predetermined temperature. Each insulator was cooled to roomtemperature, and second and third applications of the slip were usuallyapplied as described above in Example I. Test specimens on which slipswere applied and fired were placed in a sparking fixture having a headedcentral electrode, the head of which was spring biased against the endhaving the fired slip, and this assembly is then placed within a testcasing having a shoulder which abutted the outside edge of the firedslip of the specimen. A gap of approximately 0.050 inch was providedbetween the head of the center electrode of the sparking fixture, andthe edge of the shoulder of the casing body, and the test specimensinstalled with the fired slip end facing upwardly in a sparking machine.Ten drops per minute of a JP4 jet fuel was dripped onto the fired slipend of the specimen, and two sparks per second were produced across thefired slip end between the electrodes. These sparks were produced bydischarging a IO-microfarad condenser charged with 2000 volts so that 20joules of energy were dissipated during each spark.

Five test specimens prepared using the same slip described above inExample I and fired at 2500 F. gave an average service life of 57.7hours. The firing of the abovementioned slips was done in an oxidizingatmosphere so that the copper metal Was converted to a copper oxide,which at approximately 2000 F. is primarily cuprous oxide. At the abovefiring temperature, cuprous oxide is believed to react with chromicoxide and alumina to form aluminates and chromates. An examination ofthe specimen after firing showed that very little of the slip remained,and that a black penetration band of cuprous oxide had penetrated thealumina insulator body to varying depths which in some instances wasapproximately inch. A small degree of swelling of the insulator bodyoccurred and spectographic analysis showed that a large amount ofcuprous oxide was present in the surface layer of the insulator in aform combined with alumina and chromium sesquioxide. The electricalresistance acnoss the electrodes of the test fixture varied between 30and 40 thousand ohms as initially produced. The specimens were sparkedin the above-described testing machine for seven hours at roomtemperature; the specimens were thereafter baked at 1000 F. to removecarbon deposits; new electrodes were fitted onto these test specimens;and the onset voltage was thereafter determined. A failure of thespecimen was deemed to have occurred and the tests were discontinuedwhen the onset voltage required to initiate sparking exceeded 1800 voltsat a capacitance of 0.1 mfd.

The time and temperature at which an insulator body coated with a slipis fired determines the depth to which the cuprous oxide of the slipdiffuses into the insulator and determines the proportion of thealuminate that is formed relative to the nonconductive material fromwhich the insulator body is made. By controlling the time andtemperature of firing, therefore, layers of various resistances can beproduced, and if the insulator body coated With the slip is fired at atemperature below approximately 2400 F. an engobe coating is produced onthe insulator body without appreciable diffusion of the euprous oxideinto the insulator body. For most igniter applications it is desired tohave a relatively high concentration of cuprous oxide in a thin denselayer adjacent the outer surface of the insulator, so that the surfacewill have a fairly high conductivity and yet be dense and erosionresistant. It appears that appreciable diffusion of the cuprous oxideinto the insulator body to form aluminates and chromates does not occurbelow approximately 2400 F. Accordingly, upon firing below thistemperature, an engobe coating on the surface of the insulator body isproduced. Upon firing above this critical temperature, diffusion intothe insulator body takes place at a rate which increases withtemperature. A satisfactory rate of diifusion takes place at about 2500F. and too high a rate of diffusion takes place above approximately 2700F. Where an engobe coating is formed rather than an aluminate orchromate layer within the surface of the insulator body, a greatlyreduced service life of the igniter is obtained. The engobe coatingsabove referred to are relatively porous and soft and have a service lifeonly a fraction of that of an insulator body having a good dense layerof aluminates and chromates adjacent the surface of the insulator body.This was demonstrated by the following procedure, which was practicedfor purposes of comparison, and not in accordance with the invention:

A slip, hereafter called Slip No. 2, comprising 93% of copper, 5% ofchromic oxide, and 2% of kaolin was painted on an alumina test insulatorof the above-described type comprising 92% of alumina, and the insulatorbody coated with the slip was thereafter fired at 2100 F. The insulatorbody was cooled, and the process was repeated to apply two furtheradditions of the slip coating to form an engobe approximately 0.006 inchthick. Visual examination showed that the insulator had an engobecoating produced thereon with substantially no diffusion of the materialfrom which the slip was made into the 1 insulator. This insulator bodywith the engobe so formed was tested in the manner above-described, andthe inservice life for samples produced at any given time. In order thatvalid comparisons can be had, therefore, it is necessary to run controlspecimens fired at previously evaluated times and temperatures each timea series of test specimens is made. Table II gives the results ofanother series of tests of insulator bodies produced in the same mannerdescribed above for the specimens tested in Table I, but fired for thetimes and temperatures given in Table II. Table II indicates thatimproved results are obtained when the insulator bodies are fired foronly approximately seven minutes, but at a temperature of 2700Fahrenheit.

sulator body with the thus formed engobes had a service life of onlyapproximately 6 hours.

Optimum sparking life is obtained when a highly dense concentration ofelectrically conductive cuprous chromates dispersed throughoutaluminates are produced adjacent the surface of the insulator with .aminimum of diffusion of the insulator material out of the surface of theinsulator. The temperature as shown above must be high enough to producethe aluminates and chromates, and the firing time must not be so long asto allow the oxide applied to the outer surface of the insulator body tobe depleted by diffusion too far into the insulator to leave a porousregion deficient in the aluminates and chromates. This is shown by thefollowing tests. Numerous A1 0 insulators of the type above-describedwere coated with a slip comprising 88.3% of copper, 9.8% of chromicoxide, and 1.9% kaolin, hereafter designated Slip No. 3, and were heatedfor the time and temperature indicated in the following Table I. Eachinsulator was cooled, a second coating of slip was applied, and theinsulator again fired at the same temperature and for the same time aswas used to fire the first coating of slip. The process was repeated fora third time and the finished insulator body was then tested in themanner above-described. Although a considerable diiference in servicelife is obtained between specimens fired at some of the highertemperatures, a comparison of the average service life of insulatorsfired at the various times and temperatures is indicative of the servicelife to be expected for insulator bodies fired at any particular timeand temperature.

TABLE I.-THREE APPLICATIONS AND FIRINGS Hours Sparking to Failure atDifferent Firing Temperature Firing Time Min 10.7 12. 2 20. 6 27. 5 35.067. 4 34. 2 11.3 12. 3 24. 0 41.9 39. 2 71. 0 52.1 15. 2 12.8 38.1 44. 742. 2 81. 6 65.0 17. 5 17. 6 23. 0 21. 1

Avg 15.5 15. 2 27. 6 38. 0 38. 8 73.3 50. 4

Min 14 0 7.0 13.8 23. 5 35. 5 15.2 9. 8 14 0 12. 0 20. 7 33.1 51. 4 42.315.0 25. 5 11.1 55. 7 36. 2 61. 2 76. 0 71.5

Avg 17. 8 10.0 30.1 30. 9 49. 4 44. 5 32. 1

50 Min 7.0 19. 6 18. 6 15. 5 37. 6 21.0 14. 0 21. 3 12. 1 7.0 14. 0 36.0 77. 1 28. 0 10.7 12.3 21.0 (42. 0) 48. 2 57. 8 14. 0

Avg 13.0 14. 7 15.5 23. 8 40. 6 52.0 18. 7

It has been found that the consistency of the slip, the

atmosphere of the furnace and other variables affect the Other testshave shown that a single application and firing of a slip produces aservice life approximately twothirds that achieved when threeapplications and firings of the slip are used to produce the insulatorbodies.

In another series of tests it has been found to be beneficial toincorporate the same material from which the insulator is made in thecopper containing material applied to the insulator body. Cuprous oxideis very fluid at 2500 F., and the viscosity of the material applied tothe insulator is increased considerably by the addition of alumina. Theaddition of alumina and chromium sesquioxide, therefore, helps to holdthe cuprous oxide in place to promote uniform penetration of theinsulator body. In addition, alumina additions to the slip increases thedensity of materials remaining on the surface of the insulator body toincrease its resistance to erosion.

Test specimens were produced by applying coatings of suitable slips toinsulator bodies in the manner abovedescribed, firing at thetemperatures indicated in Table III for ten minutes, and repeating thecoating and firing steps for a total of three applications. The coatingswere composed of Slip No. 3 and of Slip No. 3 plus varying amounts ofalumina. The compositions used and the average service life in hours arepresented in Table IH, below:

Slip No. 2, parts, plus alumina, 20 parts- 2, 500

Cupric oxide is stable in air at temperatures up to approximately 1026C. at which temperature it dissociates to the cuprous state. Cuprousoxide is stable above 1026 C. and melts at 1235 C. Cupric oxide combineswith A1 0 to form the spinel which is stable below approximately 900 C.Above approximately 900 C., a copper aluminate is formed having theformula CuAlO It appears, therefore, that the presence of A1 0 lowersthe temperature at which transformation from cupric oxide to cuprousoxide takes place.

At temperatures above approximately 1165 C. a liquid forms whichconsists principally of Cu O. This liquid can be in equilibrium withC-uAlO depending upon the amount of A1 0 in the mixture. Aboveapproximately 1260 C. the CuAlO breaks down to A1 0 and a copper oxiderich melt. It appears, therefore, that Cu O fluxes with the alumina ofthe insulator body and 7 quickly migrates beneath the surface of theinsulator body.

Applicant has found that the presence of chromium sesquioxide when addedto the material applied to the insulator bodies stabilizes theconductive phase which is formed so that the resistance of the body soformed does not change appreciably with use at a temperature of 1600 F.It appears that chromium sesquioxide has a strong affinity for cuprousoxide, probably forming CuCrO and that the large size of the chromiumsesquioxide molecule, prevents the chromium sesquioxide from appreciablediffusion or migration into the alumina insulator body, and thereforeholds the cuprous oxide in place. It may also be that chromiumsesquioxide lowers the transition temperature of cupric oxide to cuprousoxide.

In any event, applicant has found that something more than an engobe isformed when copper or any form of copper which establishes equilibriumwith oxygen at elevated temperatures are applied to an alumina insulatorbody and fired at temperatures between approximately 2500 F. and 2700 F.in an oxidizing atmosphere for a controlled short period of time whichholds diffusion of cuprous oxide to within approximately 0.010 inch ofthe insulator surface and preferably to less than 0.005 inch of itssurface. In addition, the presence of chromium sesquioxide has beenfound to prevent appreciable change in the resistance of theelectrically conductive layer so formed at use temperatures above 1600F. and even at temperatures approaching 2000 F.

To determine the effect of chromium and aluminum, six test specimens ofthe type above-described were made of the compositions given in TableIV. In each instance 2% of Kaolin was incorporated with the percentageof Cr O or A1 indicated in the table, and the balance was copper powder.Slips of these compositions were brushed in a band around the outercylindrical surface of alumina insulator bodies of the type abovedescribed and the coated bodies were fired for ten minutes at thetemperature indicated. A second coating of the same slip was appliedover the first band and the coated body refired at the temperatureindicated, after which a third coating was applied and fired in the samemanner. The resistance was measured by clamping the treated surface fthe specimens between flat plates and measuring the resistancetherebetween with a 500 volt megger. The average resistance measured forthe six test specimens coated with each mixture are given in Table IV.

TABLE IV. RESISTANCE AFTER THIRD FIRED COAT (K-OI-IMS) FiringTemperature Engobe Addition Pure Copper. 508 236 293 1, 188 7, 944 25,721 176 158 168 444 10, 333 24, 610 149 170 111 101 3,000 8, 611 81 112104 119 5, 611 28, 2'22 53 61 85 116 221 10, 222 17 18 9 15 21 33 8 9 910 20 18 4 4 4 l1 6 9 539 204 188 1,959 8, 389 30, 277 280 224 231 2,131 11, 889 26, 833 346 208 172 454 6, 111 12, 167 213 158 142 1, 467 7,500 l 28, 705 125 117 91 3, 551 9,333 30, 555 81 74 74 3, 889 12, S6622, 333 30% Al:O 88 77 70 133 15,000 29, 722

1 Less than 6 specimens included in this average.

From the above and other tests it appears that chromium sesquioxideshould comprise between 3 and 30% by weight of the solids applied to theinsulators, preferably between and 20%, and for economic reasons mostpreferably about 10% by weight. These tests also indicate that aluminaadditions improve life of the resulting article. Improved results arehad when the alumina comprises between approximately 10% and 50% byweight of the solids. It appears that no advantage is had in using morethan approximately 30% by weight of alumina, and

optimum results appear to be had when the alumina comprisesapproximately 20% of the solid mixture. Minor percentages of othermaterials can be used without shifting the Cu O-Al O and Cu OCr O phaserelationships to harmful degrees, and a considerable amount of materialswhich do not shift phase relationships can be incorporated withoutblocking the synergistic effect. Evidence of this is bad by the largeamount of alumina which can be used in the solid materials applied tothe alumina insulator bodies without greatly reducing the utility of thefired insulator. Although alumina is not inert, it is the same materialas the material from which the insulator is made, and therefore, amongother things acts as a diluent.

It will be apparent that various changes and modifications can be madefrom the specific details disclosed and described without departing fromthe spirit of the attached claims.

What I claim is:

1. A method of producing an electrically conductive surface on aninsulator body consisting principally of alumina, said methodcomprising: bringing the surface of said insulator into contact withcuprous oxide at a temperature of at least about 2400 F., andcontrolling the time and temperature of contact between said cuprousoxide and said surface of said insulator body to provide an electricallyconductive phase beneath its surface.

2. A method of producing an electrically conductive surface on aninsulator body consisting principally of alumina, said methodcomprising: bringing the surface of said insulator into contact with amixture of materials comprising a major percentage of cuprous oxide anda minor percentage of alumina at a temperature of at least about 2400F., and controlling the time and temperature of contact between saidmixture of cuprous oxide and alumina and said surface of said insulatorbody to change the surface layer of said insulator body into anelectrically conductive layer comprising CuAlO and alumina.

3. A method of producing an electrically conductive surface on aninsulator body consisting principally of alumina, said methodcomprising: bringing the surface of said insulator into contact with amixture of materials comprising a major percentage of cuprous oxide andat least about 3 percent of chromium sesquioxide at a temperature of atleast about 2400 F., and controlling the time and temperature of contactbetween said mixture of cuprous oxide and chromium sesquioxide and saidsurface of said insulator body to change the surface layer of saidinsulator body into an electrically conductive layer of reacted cuprousoxide, alumina and chromium sesquioxide.

4. A method of producing an electrically conductive surface on aninsulator body consisting principally of alumina, said methodcomprising: bringing the surface of said insulator into contact with amixture of materials comprising a major percentage of cuprous oxide, atleast about 3 percent of chromium sesquioxide and a minor percentage ofalumina, at a temperature of at least 2400 F., and controlling the timeand temperature of contact between said mixture and said surface of saidinsulator body to change the surface layer of said insulator body intoaluminates and chromates of cuprous oxide.

5. A method of producing an electrically conductive surface on aninsulator body consisting principally of alumina, said methodcomprising: bringing the surface of said insulator into contact with amixture consisting essentially of 3 to 30 percent by weight of chromiumsesquioxide, 10 to 30 percent by weight of alumina and the balancecopper in a form which establishes equilibrium with oxygen at elevatedtemperatures at a temperature of at least about 2400 F., and controllingthe time and temperature of contact between said mixture and saidinsulator body to change the surface layer of said insulator body intochromates and aluminates of cuprous oxide without appreciable migrationof the alumina out of said insulator body.

6. A method of producing an electrically conductive surface on aninsulator body consisting principally of alumina, said methodcomprising: bringing the surface of said insulator into contact with amixture consisting essentially of 3 to 30 percent by weight of chromiumsesquioxide, to 30 percent by weight of alumina and the balance copperin a form which establishes equilibrium with oxygen at elevatedtemperatures, and heating said mixture and insulator body to atemperature between approximately 2400 F. and approximately 2700 F. fora period of time between approximately 10 minutes and approximately 6minutes to change the surface layers of said insulator body intochromates and aluminates of cuprous oxide without appreciable migrationof the alumina out of said insulator body.

7. A method of producing an electrically conductive surface On aninsulator body consisting essentially of alumina, said methodcomprising: bringing the surface of said insulator into contact withmaterial consisting essentially of approximately 90% by weight of copperin a form which establishes equilibrium with oxygen at elevatedtemperatures and approximately 10% by Weight of Cr O at a temperature ofbetween approximately 2400 F. and approximately 2700 F. for betweenapproximately ten minutes to approximately six minutes in an oxidizingatmosphere to produce an electrically semiconductive layer adjacent thesurface of said insulator.

8. A method of producing an electrically conductive surface on aninsulator body consisting essentially of alumina, said methodcomprising: bringing the surface of said insulator into contact withmaterial consisting essentially of approximately 75% by weight of copperin a form which establishes equilibrium with oxygen at elevatedtemperatures, approximately 16.5% by weight of alumina, andapproximately 8.5% by weight of chromium sesquioxide at a temperatureabove approximately 2400 F. for approximately ten minutes in anoxidizing atmosphere.

9. A method for producing an electrically conductive copper oxidecontaining surface region on an insulator consisting principally ofalumina, said method comprising: applying to a surface of the insulatora coating of a copper oxide containing composition capable of forming anelectrically semiconducting coating on the insulator, the compositionincluding at least about 3 percent of an oxide of chromium, the amountof the oxide being sufficient to stabilize the surface resistance of thecoating upon firing to temperatures above approximately 2400 F., andfiring the insulator and coating to a temperature above approximately2400 F., but not sufiiciently high that the surface resistance of thecoating is appreciably higher than the surface resistance of the coatingfired to maturation, and for a time sufiiciently short that, adjacentthe origin-a1 insulator surface and immediately therebelow there is acopper rich electrically semiconductive region which is substantiallydevoid of free alumina.

10. A new and improved insulator body having an electrically conductivesurface comprising a body of alumina fired to approximately oftheoretical density, said body having a surface layer between 0.001 inchand 0.010 inch thick containing Cu O united with the alumina of saidinsulator body to form CuAlO therewith.

11. A new and improved insulator body having an electrically conductivesurface comprising a body of alumina fired to approximately 95 oftheoretical density, said body having a surface layer between 0.001 inchand 0.010 inch thick containing cuprous oxide and chromium sesquioxideunited with the alumina of said insulator body to form a stabilizedelectrically conductive layer in said insulator body.

References Cited WILLIAM L. JARVIS, Primary Examiner.

